High pressure sodium vapor lamps utilizing a polycrystalline alumina arc tube hermetically sealed at each end with ceramic end closures of various types are already known. For example, in U.S. Pat. No. 4,442,378, assigned to the assignee of the present invention, there is described an arc tube construction wherein ceramic plugs are inserted into each end of the arc tube and hermetically sealed thereto with a sealing glass frit by conventional means. Each of said ceramic plugs further include central apertures or openings through which extend lead-in conductors that are connected to the thermionic electrodes contained wthin said arc tube. The conventional thermionic electrodes comprise refractory metal coils wound around a tungsten shank and with one of said electrodes further including a tubular metal in-lead conductor extending externally from said arc tube and containing a reservoir of sodium-mercury amalgam in excess of the quantity vaporized during lamp operation. Inert gas filling is also contained within said arc tube to facilitate lamp starting and the conventional lamp construction further includes an outer light-transmitting envelope surrounding said arc tube having a stem press seal at one end through which extends a pair of in-leads electrically connected to said thermionic electrodes.
A different type of ceramic end closure has also been used to hermetically seal one or both ends of a polycrystalline alumina arc tube in said lamps wherein a flat polycrystalline alumina disc with a contour and size enabling total insertion into the internal opening of said arc tube was directly sintered together without sealing glass frit. In said prior art arc tube construction, the plug and tube members were presintered separately in air and with said plug members thereafter being partially sintered at higher elevated temperatures sufficiently to cause shrinkage of the ceramic material. The partially sintered end plugs were then assembled into one or both ends of the presintered arc tubes for sintering together, generally in a hydrogen atmosphere, whereupon shrinkage of the arc tube around the disc contour produced the desired hermetic sealing therebetween. The manner in which said final sintering operation was carried out consisted of simply inserting the plug or plug members into the tube ends and sintering the assembly while oriented in a horizontal direction to prevent movement of the inserted plug members inside the tube before sealing together had occurred. More particularly, the conventional end plugs were inserted by hand into a longer length of the polycrystalline alumina tubing at spaced apart locations corresponding to individual arc tube lengths and with said sealed arc tubes being cut to length after said final sintering step. Uneven shrinkage often resulted during said conventional final sintering step, however, either causing the arc tube to bulge or misalignment between the disc and the arc tube to occur so that the sintered assembly could not be used in either case.
Accordingly, it would be desirable to provide an improved method and means to produce this type arc tube construction with greater reliability and to do so in a manner which does not require significant modification of the existing manufacturing process. It would be further desirable to provide said improved ceramic enclosure eliminating the customary step of hand-cutting the sealed arc tube to length before use in the final lamp manufacture. Additionally, such elimination of cutting the sealed arc tubes to length after the final sintering step further reduces manufacturing costs attributable to cleaning the cut assemblies.